Bare chords

Is the simplified form of the bare chord always works or I should also need to use the full form too ?

Learn and use both. Barr Chords are a big part of playing guitar. Some songs are unplayable on guitar without Barr Chords. Transposing to another key can be all but impossible without Barr Chords.

There are many forms of Barr Chords, the more you can pull out of you hat the better. They can be difficult to master but it is worth the effort. I spent years learning to play the full A form Barr Chord clearly and reliably. Now, I use it, (and several others) all the time.